1. Field of the Invention
This invention relates to a differential phase shifting keying demodulator.
2. Description of the Prior Art
The following description deals with a device for demodulating a differential phase shift keying signal (hereinafter referred to as a DPSK signal) which is synchronized with a video signal. FIGS. 1(a)-1(d) of the accompanying drawings illustrate the DPSK signal synchronized with a video signal. Of these drawings, FIG. 1(a) shows information data before modulation. The information data transmits the data of one bit during 4 horizontal scanning periods (hereinafter the horizontal scanning period will be called H period for short). FIG. 1(b) shows the differential data of the information data of FIG. 1(a). When the information data is "0", the differential data remains the same as the data obtained immediately before the "0" information data. When the information data is "1", the differential data is inverted from the data obtained immediately before it. FIG. 1(c) shows the video signal. FIG. 1(d) shows the DPSK signal. As is apparent from the time expanded illustration part on the right-hand side of FIGS. 1(a)-1(d), the DPSK signal of FIG. 1(d) has its phase inverted as the differential data signal of FIG. 1(b) changes between "1" and "0" while its frequency remains unchanged. Let us assume here that the DPSK signal is in a predetermined phased relation to the horizontal synchronizing signal of the video signal shown in FIG. 1(c).
Heretofore, for demodulating this type of DPSK signal, a circuit which is arranged as shown in FIG. 2 has been used. Referring to FIG. 2, the circuit includes a terminal 1 which receives the DPSK signal; a phase-locked loop (hereinafter referred to as PLL) circuit 2 produces a signal which has its phase coincident with that of the DPSK signal supplied to the terminal 1 and has a frequency twice as high as that of the DPSK signal; a 1/2 frequency divider 3; a comparator 4 to compare the DPSK signal with the output of the frequency divider 3; a demodulator 5 to obtain original information data from differential data; and an output terminal 6 to produce reproduced information data therefrom.
FIGS. 3(a)-3(e) show, in a timing chart, the wave forms of parts "a"-"e" of FIG. 2. As is apparent from FIG. 3(c), the 1/2 frequency divider 3 produces a signal which is of the same frequency as that of the DPSK signal and the signal has an unvarying phase. The comparator 4 produces a high level output when the phase difference between two input signals thereof is 180 degrees. Accordingly, the level of the output of the comparator 4 becomes high only when the DPSK signal is inverted relative to the signal produced from the 1/2 frequency divider 4 to give the above differential data of the original information data. The demodulator 5 samples the output of the comparator 4 at every 4H period, i.e. at every period during which one bit portion of data is transmitted. The demodulator 5 thus inverts the output data of the comparator 4 when the sampled output thereof is at a high level. Therefore, the original information data can be obtained from the demodulator 5.
However, in carrying out demodulation with the conventional device which is arranged as described above, the original data is demodulated after the differential data of the data is obtained. This arrangement results in a complex circuit arrangement. Furthermore, since the conventional demodulating device necessitates the use of the PLL circuit 2, the frequency divider 4, etc., this inevitably results in an increase in the scale of the circuit arrangement of the demodulating device.
It is an object of this invention to provide a demodulating device which deletes the drawback of the prior art device described above and is capable of demodulating, in an extremely simple manner, a modulated signal which has been phase shifted in relation to a differential signal of information data.
It is another object of the invention to provide a demodulating device capable of demodulating a modulated signal which has been phase shifted in relation to a differential signal of information data to directly restore the signal into information data obtained prior to modulation.
The above and further objects and features of the invention will become apparent from the following detailed description of embodiments thereof taken in conjunction with the accompanying drawings.